There are two major aims of the proposed work. The first is the elucidation of the regulatory roles of calcium and cyclic AMP as interrelated messengers in cell activation. The cellular systems under study are: a) the action of catecholamines upon cyclic nucleotide metabolism in the rat erythrocyte; b) the action of antidiuretic hormone in the toad urinary bladder; c) the role of calcium dependent regulatory protein in the control of calcium transport in the red blood cell. The second study concerns the cellular basis of action of vitamin D in the D-deficient chick duodenum and aldosterone in the rabbit colon. Particular emphasis will be placed on studies of the effect of these hormones on the structure and transport properties of isolated brush border and basolateral membrane vesicles from intestinal mucosal cells and the cells of the proximal renal tubule.